Not applicable.
The present invention relates to a multiple-nip calender, such as a supercalender, for producing a matte-quality paper web, said calender comprising a roll system composed of a plurality of rolls arranged to be in nip contact with each other or a corresponding roll stack, in which a paper web to calender the same is arranged to pass through the nips.
The roll system of a conventional supercalender comprises a number of rolls, usually eight to fourteen rolls, arranged into a superimposed roll stack. The superimposed rolls are in nip contact with each other and the paper web to be calendered is arranged to run through the nips between the rolls. Also, such supercalenders are known in the art in which the rolls of the roll system are so arranged that the nip level mainly common to the roll nips is arranged to be essentially horizontal instead of using a vertical roll stack. As regards the calendering result, it is not, however, of great importance whether a vertical or horizontal roll system is used. A highly conventional supercalender design is disclosed in, for example, Fl patent specification No. 81 633.
When wishing to raise the level of calendering with currently known designs, the sole possibility is, in fact, to increase the number of calendering nips in the supercalenders. This leads to a more complicated calender structure and to a more difficult control and tail threading of the paper web. Especially when on-line machines are in question, conflicts may be caused by the great running speed and full-speed tail threading. Endeavours have been made to solve the problems by, for example, different belt and shoe calenders, with which the calendering nip is extended and, therethrough, the activity of the nip is intensified. As to the state of art related to belt calenders, reference is made, for instance, to Finnish patent specifications Nos. 95 061, 102 304, and 102 305. As regards the state of the art related to shoe calenders, reference is made further to German application specification No. 43 44 165 and U.S. Pat. No. 5,163,364.
Using belt calenders and shoe calenders, a gloss corresponding to a supercalendered quality or fairly close to the gloss achieved by supercalendering is achieved, though achieving an equivalent smoothness is difficult. In order to avoid one-sidedness problems of the calendered paper, it is in general indispensable to use at least two extended nips provided with a belt or a shoe roll. In a belt and shoe calender nip, the maximum pressure will be lower thanks to the wider nip than in conventional nips formed between two rolls, because of which the belt and shoe calenders are best appropriate for paper grades in which the maintaining of bulk is of a great importance. They are thought to be particularly well appropriate for use in calendering cardboard.
According to the prior art, when calendering a paper web to achieve a matte-quality paper web, the calendering nip is formed between a polymer roll and a specially coated, heatable thermoroll or between two polymer rolls. The problem is now especially the short nip area in the running direction of the web involving detrimental resulting effects caused thereby in producing a matte-quality paper web.
A first objective of the present invention is to provide a novel solution in a multiple-nip calender, by which solution are provided as a combination those advantages which are related to current multiple-nip calenders and to e.g. belt and shoe calenders provided with an extended nip.
A second objective of the present invention is to provide a novel calendering arrangement for forming a calendering nip in order to make feasible production of matte-quality, that is, non-gloss and smooth paper web easier than before, especially a more easily controllable calendering when producing matte-quality paper web.
To achieve the objects of the invention, the multiple-nip calender of the invention mentioned at the beginning is mainly characterized in that at least the first and/or the last calendering nip of the multiple-nip calender is an extended nip.
With the present invention, a plurality of advantages are gained known in prior art technology, and for instance the following of said advantages may be introduced in the present context. In the invention the properties of an extended nip and a conventional supercalender are combined, whereby, especially when using an extended and low-pressure nip as the first nip of the calender, paper can be heated, plasticized and worked appropriately with this kind of extended and low-pressure nip prior to the actual nips of the supercalender. In this manner, the bulk of the paper can be saved without compromising over other quality-related characteristics. With the aid of the invention, the quality properties of superior paper are achieved in one and the same paper density. With an extended nip connected to the supercalender, the paper gloss is increased by reducing the microroughness. The gloss and smoothness of the paper are in general superior to conventional designs and the uniformity of pressed gloss is better than earlier. The calendering process is in general a highly xe2x80x9cviolentxe2x80x9d measure considering the paper, but especially when a nip extended in the manner disclosed in the invention is used as a first nip of the supercalender, it will not cause stress to the paper to the extent the conventional nips do, whereby the structure of the paper will not suffer in the calendering. The strength properties of the paper remain good. With the design of the invention, more surface-directional forces are achieved and, on the strength of said surface-directional forces and plasticizing of the web, the smoothness of the paper to be calendered also exceeds the conventional level.
Of the advantages of the invention related to producing a matte-quality paper web, one may mention that an extended nip and, as a result of calendering, an excellent and non-gloss microsmoothness of the surface are achieved. However, the quality of the calendering result is limited in that if so-called Bendsen smoothness is desired, other calendering designs have to be used.
The other advantages and characteristic features of the invention become obvious in the following detailed description of the invention.